Introduce constants for the different elements of the StarLasuLWLanguage

[ftomassetti]

It would permit to avoid code such as:

let transpiledNodeReference = node.references.find(reference => reference.reference.key === "com_strumenta_starlasu-ASTNode-transpiledNodes-key");

In Kolasu we would access this contant and get its key field:
https://github.com/Strumenta/kolasu/blob/ee6bab757be1389c2f31835bbecfcd128c09b50f/lionweb/src/main/kotlin/com/strumenta/kolasu/lionweb/StarLasuLWLanguage.kt#L170

[alessiostalla]

There is a constant for ASTNode already. However, lionweb-typescript doesn't have getReferenceByName, etc.